In recent years, efforts have been made in the field of automotive and other transport vehicle bodies to improve fuel consumption by lowering weight in order to deal with environmental problems due to exhaust gas or so. As a result lighter Al alloy materials such as rolled sheets and extruded section materials have come to be used increasingly in automobile bodies in place of conventional steel materials.
Of these, use of Al—Mg series aluminum alloy or JIS 5000 series (hereunder called simply 5000 series or Al—Mg series) aluminum alloy sheets or Al—Mg—Si series aluminum alloy or JIS 6000 series aluminum alloy sheets has been studied for outer panels, inner panels and so on of automobile body panels (panes structures) such as automobile hoods, fenders, doors, roofs and trunk lids.
The aforementioned aluminum (sometimes called Al below) alloy sheets for automobile body panels need to have high press formability. The Al—Mg series Al alloys, which have an excellent strength-ductility balance, are the best of the aforementioned Al alloys in terms of press formability.
Consequently, research has already been done into optimizing the manufacturing conditions and the components of such Al—Mg series Al alloy sheets. JIS A 5052, 5182 and the like are typical alloy compositions of Al—Mg series Al alloys. However, even such Al—Mg series Al alloys are less ductile and less formable than cold-rolled steel sheets.
However, when the Mg content of an Al—Mg series Al alloy is increased over 8% to make a high-Mg alloy, the strength-ductility balance improves. However, such an Al—Mg series alloy of high-Mg is difficult to manufacture industrially by normal manufacturing methods such as die-casting in which the cast ingot is hot rolled after being soaked. This is because the Mg segregates in the ingot during casting, and normal hot rolling produces an Al—Mg series alloy with much lower ductility, increasing the likelihood of cracks.
It is also difficult to hot roll an Al—Mg series alloy of high-Mg at low temperatures in order to avoid the temperature range at which the aforementioned cracking occurs. This is because the deformation resistance of the material of an Al—Mg series alloy material of high-Mg is much higher at such low temperatures, and there are severe limits on the size of a product that can be manufactured with current rolling machines.
Methods such as adding a third element such as Fe, Si or the like have also been proposed for increasing the allowable Mg content of Al—Mg series alloy of high-Mg. However, as the content of such third elements rises, coarse intermetallic compounds are more likely to forms reducing the ductility of the aluminum alloy sheet. Consequently, there is a limit on increasing the allowable Mg content, and it is difficult to include Mg in amounts over 8%.
Therefore, there have already been a variety of proposals for manufacturing Al—Mg series alloy sheets of high-Mg by continuous casting methods such as twin-rolling. In twin-roll continuous castings an aluminum alloy melt is injected from a refractory supply nozzle and solidified between a rotating pair of water-cooled copper casting molds (twin rolls), and then reduced and rapidly cooled between the twin rolls immediately after the aforementioned solidification to produce an aluminum alloy thin sheet. Examples of such twin-roll continuous casting methods include Hunter's methods and the 3C method.
The cooling rate in twin-roll continuous casting is 1-3 digits larger than that of conventional DC casting or continuous belt casting Consequently, the resulting aluminum alloy sheet has an extremely fire structure, and excellent workability including press formability A relatively thin aluminum alloy sheet with a thickness of 1 to 13 mm can also be obtained by casting. As a results steps such as hot rough rolling and hot finish rolling which are required for conventional DC ingots (thickness 200 to 600 mm) can be omitted. Homogenization of the ingot can also be omitted in some cases.
Examples have already been proposed in which the structure of such an Al—Mg series alloy sheet of high-Mg manufactured by twin-roll continuous casting is specified with the aim of improving formability. For example, an automobile aluminum alloy sheet with excellent mechanical properties has been proposed in which the mean size of the Al—Mg series intermetallic compounds is 10 μm or less in an Al—Mg series alloy sheet with a high-Mg content of 6 to 10% (Patent document 1 below). An aluminum alloy sheet for automobile body use has also been proposed in which the mean size of the crystalline grains is restricted to 10 to 70 μm and the number of Al—Mg series intermetallic compounds having a size of 10 μm or more is restricted to 300/mm2 or less (Patent document 2 below).
Patent document 1: Japanese Patent Application Laid-open No. H7-252571 (Claims, pages 1-2)
Patent document 2: Japanese Patent Application Laid-open No. H8-165538 (Claims, pages 1-2)